]> git.lizzy.rs Git - rust.git/blobdiff - src/bootstrap/compile.rs
Change code to work with the new system
[rust.git] / src / bootstrap / compile.rs
index 4a972ebf8df9d9532f08abd3534863db01fd3ecd..07a0f63e6cb9230b3891b99b0da70f7558169cb6 100644 (file)
 use util::{exe, libdir, is_dylib, copy};
 use {Build, Compiler, Mode};
 
-//    for (krate, path, _default) in krates("std") {
-//        rules.build(&krate.build_step, path)
-//             .dep(|s| s.name("startup-objects"))
-//             .dep(move |s| s.name("rustc").host(&build.build).target(s.host))
-//             .run(move |s| compile::std(build, s.target, &s.compiler()));
-//    }
-//    for (krate, path, _default) in krates("test") {
-//        rules.build(&krate.build_step, path)
-//             .dep(|s| s.name("libstd-link"))
-//             .run(move |s| compile::test(build, s.target, &s.compiler()));
-//    }
-//    for (krate, path, _default) in krates("rustc-main") {
-//        rules.build(&krate.build_step, path)
-//             .dep(|s| s.name("libtest-link"))
-//             .dep(move |s| s.name("llvm").host(&build.build).stage(0))
-//             .dep(|s| s.name("may-run-build-script"))
-//             .run(move |s| compile::rustc(build, s.target, &s.compiler()));
-//    }
 //
 //    // Crates which have build scripts need to rely on this rule to ensure that
 //    // the necessary prerequisites for a build script are linked and located in
@@ -147,65 +129,124 @@ fn crate_rule<'a, 'b>(build: &'a Build,
         rule
 }
 
-/// Build the standard library.
-///
-/// This will build the standard library for a particular stage of the build
-/// using the `compiler` targeting the `target` architecture. The artifacts
-/// created will also be linked into the sysroot directory.
-pub fn std(build: &Build, target: &str, compiler: &Compiler) {
-    let libdir = build.sysroot_libdir(compiler, target);
-    t!(fs::create_dir_all(&libdir));
-
-    let _folder = build.fold_output(|| format!("stage{}-std", compiler.stage));
-    println!("Building stage{} std artifacts ({} -> {})", compiler.stage,
-             compiler.host, target);
-
-    let out_dir = build.cargo_out(compiler, Mode::Libstd, target);
-    build.clear_if_dirty(&out_dir, &build.compiler_path(compiler));
-    let mut cargo = build.cargo(compiler, Mode::Libstd, target, "build");
-    let mut features = build.std_features();
-
-    if let Some(target) = env::var_os("MACOSX_STD_DEPLOYMENT_TARGET") {
-        cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
-    }
+//        rules.build("libstd", "src/libstd")
+//             .dep(|s| s.name("rustc").target(s.host))
+//             .dep(|s| s.name("libstd-link"));
+//    for (krate, path, _default) in krates("std") {
+//        rules.build(&krate.build_step, path)
+//             .dep(|s| s.name("startup-objects"))
+//             .dep(move |s| s.name("rustc").host(&build.build).target(s.host))
+//             .run(move |s| compile::std(build, s.target, &s.compiler()));
+//    }
+#[derive(Serialize)]
+pub struct Std<'a> {
+    pub target: &'a str,
+    pub compiler: &'a Compiler<'a>,
+}
+
+impl<'a> Step<'a> for Std<'a> {
+    type Output = ();
+    const DEFAULT: bool = true;
 
-    // When doing a local rebuild we tell cargo that we're stage1 rather than
-    // stage0. This works fine if the local rust and being-built rust have the
-    // same view of what the default allocator is, but fails otherwise. Since
-    // we don't have a way to express an allocator preference yet, work
-    // around the issue in the case of a local rebuild with jemalloc disabled.
-    if compiler.stage == 0 && build.local_rebuild && !build.config.use_jemalloc {
-        features.push_str(" force_alloc_system");
+    fn should_run(builder: &Builder, path: &Path) -> bool {
+        path.ends_with("src/libstd") ||
+        builder.crates("std").into_iter().any(|(_, krate_path)| {
+            path.ends_with(krate_path)
+        })
     }
 
-    if compiler.stage != 0 && build.config.sanitizers {
-        // This variable is used by the sanitizer runtime crates, e.g.
-        // rustc_lsan, to build the sanitizer runtime from C code
-        // When this variable is missing, those crates won't compile the C code,
-        // so we don't set this variable during stage0 where llvm-config is
-        // missing
-        // We also only build the runtimes when --enable-sanitizers (or its
-        // config.toml equivalent) is used
-        cargo.env("LLVM_CONFIG", build.llvm_config(target));
+    fn make_run(builder: &Builder, _path: Option<&Path>, host: &str, target: &str) {
+        builder.ensure(Std {
+            compiler: builder.compiler(builder.top_stage, host),
+            target,
+        })
     }
-    cargo.arg("--features").arg(features)
-         .arg("--manifest-path")
-         .arg(build.src.join("src/libstd/Cargo.toml"));
 
-    if let Some(target) = build.config.target_config.get(target) {
-        if let Some(ref jemalloc) = target.jemalloc {
-            cargo.env("JEMALLOC_OVERRIDE", jemalloc);
+    /// Build the standard library.
+    ///
+    /// This will build the standard library for a particular stage of the build
+    /// using the `compiler` targeting the `target` architecture. The artifacts
+    /// created will also be linked into the sysroot directory.
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let target = self.target;
+        let compiler = self.compiler;
+
+        builder.ensure(StartupObjects { compiler, target });
+
+        if build.force_use_stage1(compiler, target) {
+            let from = builder.compiler(1, &build.build);
+            builder.ensure(Std {
+                compiler: from,
+                target: target,
+            });
+            println!("Uplifting stage1 std ({} -> {})", from.host, target);
+            builder.ensure(StdLink {
+                compiler: from,
+                target_compiler: compiler,
+                target: target,
+            });
+            return;
         }
-    }
-    if target.contains("musl") {
-        if let Some(p) = build.musl_root(target) {
-            cargo.env("MUSL_ROOT", p);
+
+        let _folder = build.fold_output(|| format!("stage{}-std", compiler.stage));
+        println!("Building stage{} std artifacts ({} -> {})", compiler.stage,
+                compiler.host, target);
+
+        let out_dir = build.cargo_out(compiler, Mode::Libstd, target);
+        build.clear_if_dirty(&out_dir, &build.compiler_path(compiler));
+        let mut cargo = builder.cargo(compiler, Mode::Libstd, target, "build");
+        let mut features = build.std_features();
+
+        if let Some(target) = env::var_os("MACOSX_STD_DEPLOYMENT_TARGET") {
+            cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
         }
-    }
 
-    run_cargo(build,
-              &mut cargo,
-              &libstd_stamp(build, &compiler, target));
+        // When doing a local rebuild we tell cargo that we're stage1 rather than
+        // stage0. This works fine if the local rust and being-built rust have the
+        // same view of what the default allocator is, but fails otherwise. Since
+        // we don't have a way to express an allocator preference yet, work
+        // around the issue in the case of a local rebuild with jemalloc disabled.
+        if compiler.stage == 0 && build.local_rebuild && !build.config.use_jemalloc {
+            features.push_str(" force_alloc_system");
+        }
+
+        if compiler.stage != 0 && build.config.sanitizers {
+            // This variable is used by the sanitizer runtime crates, e.g.
+            // rustc_lsan, to build the sanitizer runtime from C code
+            // When this variable is missing, those crates won't compile the C code,
+            // so we don't set this variable during stage0 where llvm-config is
+            // missing
+            // We also only build the runtimes when --enable-sanitizers (or its
+            // config.toml equivalent) is used
+            cargo.env("LLVM_CONFIG", build.llvm_config(target));
+        }
+
+        cargo.arg("--features").arg(features)
+            .arg("--manifest-path")
+            .arg(build.src.join("src/libstd/Cargo.toml"));
+
+        if let Some(target) = build.config.target_config.get(target) {
+            if let Some(ref jemalloc) = target.jemalloc {
+                cargo.env("JEMALLOC_OVERRIDE", jemalloc);
+            }
+        }
+        if target.contains("musl") {
+            if let Some(p) = build.musl_root(target) {
+                cargo.env("MUSL_ROOT", p);
+            }
+        }
+
+        run_cargo(build,
+                &mut cargo,
+                &libstd_stamp(build, &compiler, target));
+
+        builder.ensure(StdLink {
+            compiler: builder.compiler(compiler.stage, &build.build),
+            target_compiler: compiler,
+            target: target,
+        });
+    }
 }
 
 
@@ -216,36 +257,49 @@ pub fn std(build: &Build, target: &str, compiler: &Compiler) {
 //            compile::std_link)
 //     .dep(|s| s.name("startup-objects"))
 //     .dep(|s| s.name("create-sysroot").target(s.host));
-/// Link all libstd rlibs/dylibs into the sysroot location.
-///
-/// Links those artifacts generated by `compiler` to a the `stage` compiler's
-/// sysroot for the specified `host` and `target`.
-///
-/// Note that this assumes that `compiler` has already generated the libstd
-/// libraries for `target`, and this method will find them in the relevant
-/// output directory.
-pub fn std_link(build: &Build,
-                compiler: &Compiler,
-                target_compiler: &Compiler,
-                target: &str) {
-    println!("Copying stage{} std from stage{} ({} -> {} / {})",
-             target_compiler.stage,
-             compiler.stage,
-             compiler.host,
-             target_compiler.host,
-             target);
-    let libdir = build.sysroot_libdir(target_compiler, target);
-    add_to_sysroot(&libdir, &libstd_stamp(build, compiler, target));
-
-    if target.contains("musl") && !target.contains("mips") {
-        copy_musl_third_party_objects(build, target, &libdir);
-    }
 
-    if build.config.sanitizers && compiler.stage != 0 && target == "x86_64-apple-darwin" {
-        // The sanitizers are only built in stage1 or above, so the dylibs will
-        // be missing in stage0 and causes panic. See the `std()` function above
-        // for reason why the sanitizers are not built in stage0.
-        copy_apple_sanitizer_dylibs(&build.native_dir(target), "osx", &libdir);
+#[derive(Serialize)]
+struct StdLink<'a> {
+    pub compiler: Compiler<'a>,
+    pub target_compiler: Compiler<'a>,
+    pub target: &'a str,
+}
+
+impl<'a> Step<'a> for StdLink<'a> {
+    type Output = ();
+
+    /// Link all libstd rlibs/dylibs into the sysroot location.
+    ///
+    /// Links those artifacts generated by `compiler` to a the `stage` compiler's
+    /// sysroot for the specified `host` and `target`.
+    ///
+    /// Note that this assumes that `compiler` has already generated the libstd
+    /// libraries for `target`, and this method will find them in the relevant
+    /// output directory.
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let compiler = self.compiler;
+        let target_compiler = self.target_compiler;
+        let target = self.target;
+        println!("Copying stage{} std from stage{} ({} -> {} / {})",
+                target_compiler.stage,
+                compiler.stage,
+                compiler.host,
+                target_compiler.host,
+                target);
+        let libdir = build.sysroot_libdir(target_compiler, target);
+        add_to_sysroot(&libdir, &libstd_stamp(build, compiler, target));
+
+        if target.contains("musl") && !target.contains("mips") {
+            copy_musl_third_party_objects(build, target, &libdir);
+        }
+
+        if build.config.sanitizers && compiler.stage != 0 && target == "x86_64-apple-darwin" {
+            // The sanitizers are only built in stage1 or above, so the dylibs will
+            // be missing in stage0 and causes panic. See the `std()` function above
+            // for reason why the sanitizers are not built in stage0.
+            copy_apple_sanitizer_dylibs(&build.native_dir(target), "osx", &libdir);
+        }
     }
 }
 
@@ -274,66 +328,146 @@ fn copy_apple_sanitizer_dylibs(native_dir: &Path, platform: &str, into: &Path) {
 //      .dep(|s| s.name("create-sysroot").target(s.host))
 //      .run(move |s| compile::build_startup_objects(build, &s.compiler(), s.target));
 
-/// Build and prepare startup objects like rsbegin.o and rsend.o
-///
-/// These are primarily used on Windows right now for linking executables/dlls.
-/// They don't require any library support as they're just plain old object
-/// files, so we just use the nightly snapshot compiler to always build them (as
-/// no other compilers are guaranteed to be available).
-pub fn build_startup_objects(build: &Build, for_compiler: &Compiler, target: &str) {
-    if !target.contains("pc-windows-gnu") {
-        return
+#[derive(Serialize)]
+pub struct StartupObjects<'a> {
+    pub for_compiler: Compiler<'a>,
+    pub target: &'a str,
+}
+
+impl<'a> Step<'a> for StartupObjects<'a> {
+    type Output = ();
+
+    fn should_run(_builder: &Builder, path: &Path) -> bool {
+        path.ends_with("src/rtstartup")
     }
 
-    let compiler = Compiler::new(0, &build.build);
-    let compiler_path = build.compiler_path(&compiler);
-    let src_dir = &build.src.join("src/rtstartup");
-    let dst_dir = &build.native_dir(target).join("rtstartup");
-    let sysroot_dir = &build.sysroot_libdir(for_compiler, target);
-    t!(fs::create_dir_all(dst_dir));
-    t!(fs::create_dir_all(sysroot_dir));
-
-    for file in &["rsbegin", "rsend"] {
-        let src_file = &src_dir.join(file.to_string() + ".rs");
-        let dst_file = &dst_dir.join(file.to_string() + ".o");
-        if !up_to_date(src_file, dst_file) {
-            let mut cmd = Command::new(&compiler_path);
-            build.run(cmd.env("RUSTC_BOOTSTRAP", "1")
-                        .arg("--cfg").arg(format!("stage{}", compiler.stage))
-                        .arg("--target").arg(target)
-                        .arg("--emit=obj")
-                        .arg("--out-dir").arg(dst_dir)
-                        .arg(src_file));
-        }
-
-        copy(dst_file, &sysroot_dir.join(file.to_string() + ".o"));
+    fn make_run(builder: &Builder, _path: Option<&Path>, host: &str, target: &str) {
+        builder.ensure(StartupObjects {
+            compiler: builder.compiler(builder.top_stage, host),
+            target,
+        })
     }
 
-    for obj in ["crt2.o", "dllcrt2.o"].iter() {
-        copy(&compiler_file(build.cc(target), obj), &sysroot_dir.join(obj));
+    /// Build and prepare startup objects like rsbegin.o and rsend.o
+    ///
+    /// These are primarily used on Windows right now for linking executables/dlls.
+    /// They don't require any library support as they're just plain old object
+    /// files, so we just use the nightly snapshot compiler to always build them (as
+    /// no other compilers are guaranteed to be available).
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let for_compiler = self.for_compiler;
+        let target = self.target;
+        if !target.contains("pc-windows-gnu") {
+            return
+        }
+
+        let compiler = Compiler::new(0, &build.build);
+        let compiler_path = build.compiler_path(&compiler);
+        let src_dir = &build.src.join("src/rtstartup");
+        let dst_dir = &build.native_dir(target).join("rtstartup");
+        let sysroot_dir = &build.sysroot_libdir(for_compiler, target);
+        t!(fs::create_dir_all(dst_dir));
+        t!(fs::create_dir_all(sysroot_dir));
+
+        for file in &["rsbegin", "rsend"] {
+            let src_file = &src_dir.join(file.to_string() + ".rs");
+            let dst_file = &dst_dir.join(file.to_string() + ".o");
+            if !up_to_date(src_file, dst_file) {
+                let mut cmd = Command::new(&compiler_path);
+                build.run(cmd.env("RUSTC_BOOTSTRAP", "1")
+                            .arg("--cfg").arg(format!("stage{}", compiler.stage))
+                            .arg("--target").arg(target)
+                            .arg("--emit=obj")
+                            .arg("--out-dir").arg(dst_dir)
+                            .arg(src_file));
+            }
+
+            copy(dst_file, &sysroot_dir.join(file.to_string() + ".o"));
+        }
+
+        for obj in ["crt2.o", "dllcrt2.o"].iter() {
+            copy(&compiler_file(build.cc(target), obj), &sysroot_dir.join(obj));
+        }
     }
 }
 
-/// Build libtest.
-///
-/// This will build libtest and supporting libraries for a particular stage of
-/// the build using the `compiler` targeting the `target` architecture. The
-/// artifacts created will also be linked into the sysroot directory.
-pub fn test(build: &Build, target: &str, compiler: &Compiler) {
-    let _folder = build.fold_output(|| format!("stage{}-test", compiler.stage));
-    println!("Building stage{} test artifacts ({} -> {})", compiler.stage,
-             compiler.host, target);
-    let out_dir = build.cargo_out(compiler, Mode::Libtest, target);
-    build.clear_if_dirty(&out_dir, &libstd_stamp(build, compiler, target));
-    let mut cargo = build.cargo(compiler, Mode::Libtest, target, "build");
-    if let Some(target) = env::var_os("MACOSX_STD_DEPLOYMENT_TARGET") {
-        cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
+//    for (krate, path, _default) in krates("test") {
+//        rules.build(&krate.build_step, path)
+//             .dep(|s| s.name("libstd-link"))
+//             .run(move |s| compile::test(build, s.target, &s.compiler()));
+//    }
+#[derive(Serialize)]
+pub struct Test<'a> {
+    pub compiler: Compiler<'a>,
+    pub target: &'a str,
+}
+
+impl<'a> Step<'a> for Test<'a> {
+    type Output = ();
+    const DEFAULT: bool = true;
+
+    fn should_run(builder: &Builder, path: &Path) -> bool {
+        path.ends_with("src/libtest") ||
+        builder.crates("test").into_iter().any(|(_, krate_path)| {
+            path.ends_with(krate_path)
+        })
+    }
+
+    fn make_run(builder: &Builder, _path: Option<&Path>, host: &str, target: &str) {
+        builder.ensure(Test {
+            compiler: builder.compiler(builder.top_stage, host),
+            target,
+        })
+    }
+
+    /// Build libtest.
+    ///
+    /// This will build libtest and supporting libraries for a particular stage of
+    /// the build using the `compiler` targeting the `target` architecture. The
+    /// artifacts created will also be linked into the sysroot directory.
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let target = self.target;
+        let compiler = self.compiler;
+
+        builder.ensure(Std { compiler, target });
+
+        if build.force_use_stage1(compiler, target) {
+            builder.ensure(Test {
+                compiler: builder.compiler(1, &build.build),
+                target: target,
+            });
+            println!("Uplifting stage1 test ({} -> {})", &build.build, target);
+            builder.ensure(TestLink {
+                compiler: builder.compiler(1, &build.build),
+                target_compiler: compiler,
+                target: target,
+            });
+            return;
+        }
+
+        let _folder = build.fold_output(|| format!("stage{}-test", compiler.stage));
+        println!("Building stage{} test artifacts ({} -> {})", compiler.stage,
+                compiler.host, target);
+        let out_dir = build.cargo_out(compiler, Mode::Libtest, target);
+        build.clear_if_dirty(&out_dir, &libstd_stamp(build, compiler, target));
+        let mut cargo = build.cargo(compiler, Mode::Libtest, target, "build");
+        if let Some(target) = env::var_os("MACOSX_STD_DEPLOYMENT_TARGET") {
+            cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
+        }
+        cargo.arg("--manifest-path")
+            .arg(build.src.join("src/libtest/Cargo.toml"));
+        run_cargo(build,
+                &mut cargo,
+                &libtest_stamp(build, compiler, target));
+
+        builder.ensure(TestLink {
+            compiler: builder.compiler(1, &build.build),
+            target_compiler: compiler,
+            target: target,
+        });
     }
-    cargo.arg("--manifest-path")
-         .arg(build.src.join("src/libtest/Cargo.toml"));
-    run_cargo(build,
-              &mut cargo,
-              &libtest_stamp(build, compiler, target));
 }
 
 
@@ -344,98 +478,180 @@ pub fn test(build: &Build, target: &str, compiler: &Compiler) {
 //            compile::test_link)
 //     .dep(|s| s.name("libstd-link"));
 
-/// Same as `std_link`, only for libtest
-pub fn test_link(build: &Build,
-                 compiler: &Compiler,
-                 target_compiler: &Compiler,
-                 target: &str) {
-    println!("Copying stage{} test from stage{} ({} -> {} / {})",
-             target_compiler.stage,
-             compiler.stage,
-             compiler.host,
-             target_compiler.host,
-             target);
-    add_to_sysroot(&build.sysroot_libdir(target_compiler, target),
-                   &libtest_stamp(build, compiler, target));
+#[derive(Serialize)]
+pub struct TestLink<'a> {
+    pub compiler: Compiler<'a>,
+    pub target_compiler: Compiler<'a>,
+    pub target: &'a str,
 }
 
-/// Build the compiler.
-///
-/// This will build the compiler for a particular stage of the build using
-/// the `compiler` targeting the `target` architecture. The artifacts
-/// created will also be linked into the sysroot directory.
-pub fn rustc(build: &Build, target: &str, compiler: &Compiler) {
-    let _folder = build.fold_output(|| format!("stage{}-rustc", compiler.stage));
-    println!("Building stage{} compiler artifacts ({} -> {})",
-             compiler.stage, compiler.host, target);
-
-    let out_dir = build.cargo_out(compiler, Mode::Librustc, target);
-    build.clear_if_dirty(&out_dir, &libtest_stamp(build, compiler, target));
-
-    let mut cargo = build.cargo(compiler, Mode::Librustc, target, "build");
-    cargo.arg("--features").arg(build.rustc_features())
-         .arg("--manifest-path")
-         .arg(build.src.join("src/rustc/Cargo.toml"));
-
-    // Set some configuration variables picked up by build scripts and
-    // the compiler alike
-    cargo.env("CFG_RELEASE", build.rust_release())
-         .env("CFG_RELEASE_CHANNEL", &build.config.channel)
-         .env("CFG_VERSION", build.rust_version())
-         .env("CFG_PREFIX", build.config.prefix.clone().unwrap_or_default());
-
-    if compiler.stage == 0 {
-        cargo.env("CFG_LIBDIR_RELATIVE", "lib");
-    } else {
-        let libdir_relative = build.config.libdir_relative.clone().unwrap_or(PathBuf::from("lib"));
-        cargo.env("CFG_LIBDIR_RELATIVE", libdir_relative);
+impl<'a> Step<'a> for Step<'a> {
+    type Output = ();
+
+    /// Same as `std_link`, only for libtest
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let compiler = self.compiler;
+        let target_compiler = self.target_compiler;
+        let target = self.target;
+        println!("Copying stage{} test from stage{} ({} -> {} / {})",
+                target_compiler.stage,
+                compiler.stage,
+                compiler.host,
+                target_compiler.host,
+                target);
+        add_to_sysroot(&build.sysroot_libdir(target_compiler, target),
+                    &libtest_stamp(build, compiler, target));
     }
+}
 
-    // If we're not building a compiler with debugging information then remove
-    // these two env vars which would be set otherwise.
-    if build.config.rust_debuginfo_only_std {
-        cargo.env_remove("RUSTC_DEBUGINFO");
-        cargo.env_remove("RUSTC_DEBUGINFO_LINES");
-    }
+//    for (krate, path, _default) in krates("rustc-main") {
+//        rules.build(&krate.build_step, path)
+//             .dep(|s| s.name("libtest-link"))
+//             .dep(move |s| s.name("llvm").host(&build.build).stage(0))
+//             .dep(|s| s.name("may-run-build-script"))
+//             .run(move |s| compile::rustc(build, s.target, &s.compiler()));
+//    }
 
-    if let Some(ref ver_date) = build.rust_info.commit_date() {
-        cargo.env("CFG_VER_DATE", ver_date);
-    }
-    if let Some(ref ver_hash) = build.rust_info.sha() {
-        cargo.env("CFG_VER_HASH", ver_hash);
-    }
-    if !build.unstable_features() {
-        cargo.env("CFG_DISABLE_UNSTABLE_FEATURES", "1");
-    }
-    // Flag that rust llvm is in use
-    if build.is_rust_llvm(target) {
-        cargo.env("LLVM_RUSTLLVM", "1");
-    }
-    cargo.env("LLVM_CONFIG", build.llvm_config(target));
-    let target_config = build.config.target_config.get(target);
-    if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
-        cargo.env("CFG_LLVM_ROOT", s);
-    }
-    // Building with a static libstdc++ is only supported on linux right now,
-    // not for MSVC or macOS
-    if build.config.llvm_static_stdcpp &&
-       !target.contains("windows") &&
-       !target.contains("apple") {
-        cargo.env("LLVM_STATIC_STDCPP",
-                  compiler_file(build.cxx(target).unwrap(), "libstdc++.a"));
-    }
-    if build.config.llvm_link_shared {
-        cargo.env("LLVM_LINK_SHARED", "1");
+#[derive(Serialize)]
+pub struct Rustc<'a> {
+    pub compiler: Compiler<'a>,
+    pub target: &'a str,
+}
+
+impl<'a> Step<'a> for Rustc<'a> {
+    type Output = ();
+    const ONLY_HOSTS: bool = true;
+    const DEFAULT: bool = true;
+
+    fn should_run(builder: &Builder, path: &Path) -> bool {
+        path.ends_with("src/librustc") ||
+        builder.crates("rustc-main").into_iter().any(|(_, krate_path)| {
+            path.ends_with(krate_path)
+        })
     }
-    if let Some(ref s) = build.config.rustc_default_linker {
-        cargo.env("CFG_DEFAULT_LINKER", s);
+
+    fn make_run(builder: &Builder, _path: Option<&Path>, host: &str, target: &str) {
+        builder.ensure(Rustc {
+            compiler: builder.compiler(builder.top_stage, host),
+            target,
+        })
     }
-    if let Some(ref s) = build.config.rustc_default_ar {
-        cargo.env("CFG_DEFAULT_AR", s);
+
+    /// Build the compiler.
+    ///
+    /// This will build the compiler for a particular stage of the build using
+    /// the `compiler` targeting the `target` architecture. The artifacts
+    /// created will also be linked into the sysroot directory.
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let compiler = self.compiler;
+        let target = self.target;
+
+        builder.ensure(Test { compiler, target });
+
+        // Build LLVM for our target. This will implicitly build the host LLVM
+        // if necessary.
+        builder.ensure(native::Llvm { target });
+
+        if build.force_use_stage1(compiler, target) {
+            builder.ensure(Rustc {
+                compiler: builder.compiler(1, &build.build),
+                target: target,
+            });
+            println!("Uplifting stage1 rustc ({} -> {})", &build.build, target);
+            builder.ensure(RustcLink {
+                compiler: builder.compiler(1, &build.build),
+                target_compiler: compiler,
+                target,
+            });
+            return;
+        }
+
+        // Ensure that build scripts have a std to link against.
+        builder.ensure(Std {
+            compiler: builder.compiler(self.compiler.stage, &build.build),
+            target: &build.build,
+        });
+
+        let _folder = build.fold_output(|| format!("stage{}-rustc", compiler.stage));
+        println!("Building stage{} compiler artifacts ({} -> {})",
+                 compiler.stage, compiler.host, target);
+
+        let out_dir = build.cargo_out(compiler, Mode::Librustc, target);
+        build.clear_if_dirty(&out_dir, &libtest_stamp(build, compiler, target));
+
+        let mut cargo = build.cargo(compiler, Mode::Librustc, target, "build");
+        cargo.arg("--features").arg(build.rustc_features())
+             .arg("--manifest-path")
+             .arg(build.src.join("src/rustc/Cargo.toml"));
+
+        // Set some configuration variables picked up by build scripts and
+        // the compiler alike
+        cargo.env("CFG_RELEASE", build.rust_release())
+             .env("CFG_RELEASE_CHANNEL", &build.config.channel)
+             .env("CFG_VERSION", build.rust_version())
+             .env("CFG_PREFIX", build.config.prefix.clone().unwrap_or_default());
+
+        if compiler.stage == 0 {
+            cargo.env("CFG_LIBDIR_RELATIVE", "lib");
+        } else {
+            let libdir_relative = build.config.libdir_relative.clone().unwrap_or(PathBuf::from("lib"));
+            cargo.env("CFG_LIBDIR_RELATIVE", libdir_relative);
+        }
+
+        // If we're not building a compiler with debugging information then remove
+        // these two env vars which would be set otherwise.
+        if build.config.rust_debuginfo_only_std {
+            cargo.env_remove("RUSTC_DEBUGINFO");
+            cargo.env_remove("RUSTC_DEBUGINFO_LINES");
+        }
+
+        if let Some(ref ver_date) = build.rust_info.commit_date() {
+            cargo.env("CFG_VER_DATE", ver_date);
+        }
+        if let Some(ref ver_hash) = build.rust_info.sha() {
+            cargo.env("CFG_VER_HASH", ver_hash);
+        }
+        if !build.unstable_features() {
+            cargo.env("CFG_DISABLE_UNSTABLE_FEATURES", "1");
+        }
+        // Flag that rust llvm is in use
+        if build.is_rust_llvm(target) {
+            cargo.env("LLVM_RUSTLLVM", "1");
+        }
+        cargo.env("LLVM_CONFIG", build.llvm_config(target));
+        let target_config = build.config.target_config.get(target);
+        if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
+            cargo.env("CFG_LLVM_ROOT", s);
+        }
+        // Building with a static libstdc++ is only supported on linux right now,
+        // not for MSVC or macOS
+        if build.config.llvm_static_stdcpp &&
+           !target.contains("windows") &&
+           !target.contains("apple") {
+            cargo.env("LLVM_STATIC_STDCPP",
+                      compiler_file(build.cxx(target).unwrap(), "libstdc++.a"));
+        }
+        if build.config.llvm_link_shared {
+            cargo.env("LLVM_LINK_SHARED", "1");
+        }
+        if let Some(ref s) = build.config.rustc_default_linker {
+            cargo.env("CFG_DEFAULT_LINKER", s);
+        }
+        if let Some(ref s) = build.config.rustc_default_ar {
+            cargo.env("CFG_DEFAULT_AR", s);
+        }
+        run_cargo(build,
+                  &mut cargo,
+                  &librustc_stamp(build, compiler, target));
+
+        builder.ensure(RustcLink {
+            compiler: builder.compiler(compiler.stage, &build.build),
+            target_compiler: compiler,
+            target,
+        });
     }
-    run_cargo(build,
-              &mut cargo,
-              &librustc_stamp(build, compiler, target));
 }
 
 // crate_rule(build,
@@ -444,36 +660,48 @@ pub fn rustc(build: &Build, target: &str, compiler: &Compiler) {
 //            "build-crate-rustc-main",
 //            compile::rustc_link)
 //     .dep(|s| s.name("libtest-link"));
-/// Same as `std_link`, only for librustc
-pub fn rustc_link(build: &Build,
-                  compiler: &Compiler,
-                  target_compiler: &Compiler,
-                  target: &str) {
-    println!("Copying stage{} rustc from stage{} ({} -> {} / {})",
-             target_compiler.stage,
-             compiler.stage,
-             compiler.host,
-             target_compiler.host,
-             target);
-    add_to_sysroot(&build.sysroot_libdir(target_compiler, target),
-                   &librustc_stamp(build, compiler, target));
+#[derive(Serialize)]
+struct RustcLink<'a> {
+    pub compiler: Compiler<'a>,
+    pub target_compiler: Compiler<'a>,
+    pub target: &'a str,
+}
+
+impl<'a> Step<'a> for RustcLink<'a> {
+    type Output = ();
+
+    /// Same as `std_link`, only for librustc
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let compiler = self.compiler;
+        let target_compiler = self.target_compiler;
+        let target = self.target;
+        println!("Copying stage{} rustc from stage{} ({} -> {} / {})",
+                 target_compiler.stage,
+                 compiler.stage,
+                 compiler.host,
+                 target_compiler.host,
+                 target);
+        add_to_sysroot(&build.sysroot_libdir(target_compiler, target),
+                       &librustc_stamp(build, compiler, target));
+    }
 }
 
 /// Cargo's output path for the standard library in a given stage, compiled
 /// by a particular compiler for the specified target.
-fn libstd_stamp(build: &Build, compiler: &Compiler, target: &str) -> PathBuf {
+pub fn libstd_stamp(build: &Build, compiler: &Compiler, target: &str) -> PathBuf {
     build.cargo_out(compiler, Mode::Libstd, target).join(".libstd.stamp")
 }
 
 /// Cargo's output path for libtest in a given stage, compiled by a particular
 /// compiler for the specified target.
-fn libtest_stamp(build: &Build, compiler: &Compiler, target: &str) -> PathBuf {
+pub fn libtest_stamp(build: &Build, compiler: &Compiler, target: &str) -> PathBuf {
     build.cargo_out(compiler, Mode::Libtest, target).join(".libtest.stamp")
 }
 
 /// Cargo's output path for librustc in a given stage, compiled by a particular
 /// compiler for the specified target.
-fn librustc_stamp(build: &Build, compiler: &Compiler, target: &str) -> PathBuf {
+pub fn librustc_stamp(build: &Build, compiler: &Compiler, target: &str) -> PathBuf {
     build.cargo_out(compiler, Mode::Librustc, target).join(".librustc.stamp")
 }
 
@@ -485,10 +713,33 @@ fn compiler_file(compiler: &Path, file: &str) -> PathBuf {
 
 // rules.build("create-sysroot", "path/to/nowhere")
 //      .run(move |s| compile::create_sysroot(build, &s.compiler()));
-pub fn create_sysroot(build: &Build, compiler: &Compiler) {
-    let sysroot = build.sysroot(compiler);
-    let _ = fs::remove_dir_all(&sysroot);
-    t!(fs::create_dir_all(&sysroot));
+
+#[derive(Serialize)]
+pub struct Sysroot<'a> {
+    pub compiler: Compiler<'a>,
+}
+
+impl<'a> Step<'a> for Sysroot<'a> {
+    type Output = PathBuf;
+
+    /// Returns the sysroot for the `compiler` specified that *this build system
+    /// generates*.
+    ///
+    /// That is, the sysroot for the stage0 compiler is not what the compiler
+    /// thinks it is by default, but it's the same as the default for stages
+    /// 1-3.
+    fn run(self, builder: &Builder) -> PathBuf {
+        let build = builder.build;
+        let compiler = self.compiler;
+        let sysroot = if compiler.stage == 0 {
+            build.out.join(compiler.host).join("stage0-sysroot")
+        } else {
+            build.out.join(compiler.host).join(format!("stage{}", compiler.stage))
+        };
+        let _ = fs::remove_dir_all(&sysroot);
+        t!(fs::create_dir_all(&sysroot));
+        sysroot
+    }
 }
 
 // the compiler with no target libraries ready to go
@@ -504,54 +755,99 @@ pub fn create_sysroot(build: &Build, compiler: &Compiler) {
 //          }
 //      })
 //      .run(move |s| compile::assemble_rustc(build, s.stage, s.target));
-/// Prepare a new compiler from the artifacts in `stage`
-///
-/// This will assemble a compiler in `build/$host/stage$stage`. The compiler
-/// must have been previously produced by the `stage - 1` build.build
-/// compiler.
-pub fn assemble_rustc(build: &Build, stage: u32, host: &str) {
-    // nothing to do in stage0
-    if stage == 0 {
-        return
-    }
 
-    println!("Copying stage{} compiler ({})", stage, host);
+#[derive(Serialize)]
+pub struct Assemble<'a> {
+    /// The compiler which we will produce in this step. Assemble itself will
+    /// take care of ensuring that the necessary prerequisites to do so exist,
+    /// that is, this target can be a stage2 compiler and Assemble will build
+    /// previous stages for you.
+    pub target_compiler: Compiler<'a>,
+}
 
-    // The compiler that we're assembling
-    let target_compiler = Compiler::new(stage, host);
+impl<'a> Step<'a> for Assemble<'a> {
+    type Output = ();
+
+    /// Prepare a new compiler from the artifacts in `stage`
+    ///
+    /// This will assemble a compiler in `build/$host/stage$stage`. The compiler
+    /// must have been previously produced by the `stage - 1` build.build
+    /// compiler.
+    fn run(self, builder: &Builder) {
+        let build = builder.build;
+        let target_compiler = self.target_compiler;
+
+        if target_compiler.stage == 0 {
+            assert_eq!(build.build, target_compiler.host,
+                "Cannot obtain compiler for non-native build triple at stage 0");
+            // The stage 0 compiler for the build triple is always pre-built.
+            return target_compiler;
+        }
 
-    // The compiler that compiled the compiler we're assembling
-    let build_compiler = Compiler::new(stage - 1, &build.build);
+        // Get the compiler that we'll use to bootstrap ourselves.
+        let build_compiler = if target_compiler.host != build.build {
+            // Build a compiler for the host platform. We cannot use the stage0
+            // compiler for the host platform for this because it doesn't have
+            // the libraries we need.  FIXME: Perhaps we should download those
+            // libraries? It would make builds faster...
+            builder.ensure(Assemble {
+                target_compiler: Compiler {
+                    // FIXME: It may be faster if we build just a stage 1
+                    // compiler and then use that to bootstrap this compiler
+                    // forward.
+                    stage: target_compiler.stage - 1,
+                    host: &build.build
+                },
+            })
+        } else {
+            // Build the compiler we'll use to build the stage requested. This
+            // may build more than one compiler (going down to stage 0).
+            builder.ensure(Assemble {
+                target_compiler: target_compiler.with_stage(target_compiler.stage - 1),
+            })
+        };
 
-    // Link in all dylibs to the libdir
-    let sysroot = build.sysroot(&target_compiler);
-    let sysroot_libdir = sysroot.join(libdir(host));
-    t!(fs::create_dir_all(&sysroot_libdir));
-    let src_libdir = build.sysroot_libdir(&build_compiler, host);
-    for f in t!(fs::read_dir(&src_libdir)).map(|f| t!(f)) {
-        let filename = f.file_name().into_string().unwrap();
-        if is_dylib(&filename) {
-            copy(&f.path(), &sysroot_libdir.join(&filename));
+        // Build the libraries for this compiler to link to (i.e., the libraries
+        // it uses at runtime). NOTE: Crates the target compiler compiles don't
+        // link to these. (FIXME: Is that correct? It seems to be correct most
+        // of the time but I think we do link to these for stage2/bin compilers
+        // when not performing a full bootstrap).
+        builder.ensure(Rustc { compiler: build_compiler, target: target_compiler.host });
+
+        let stage = target_compiler.stage;
+        let host = target_compiler.host;
+        println!("Assembling stage{} compiler ({})", stage, host);
+
+        // Link in all dylibs to the libdir
+        let sysroot = build.sysroot(&target_compiler);
+        let sysroot_libdir = sysroot.join(libdir(host));
+        t!(fs::create_dir_all(&sysroot_libdir));
+        let src_libdir = build.sysroot_libdir(&build_compiler, host);
+        for f in t!(fs::read_dir(&src_libdir)).map(|f| t!(f)) {
+            let filename = f.file_name().into_string().unwrap();
+            if is_dylib(&filename) {
+                copy(&f.path(), &sysroot_libdir.join(&filename));
+            }
         }
-    }
 
-    let out_dir = build.cargo_out(&build_compiler, Mode::Librustc, host);
-
-    // Link the compiler binary itself into place
-    let rustc = out_dir.join(exe("rustc", host));
-    let bindir = sysroot.join("bin");
-    t!(fs::create_dir_all(&bindir));
-    let compiler = build.compiler_path(&target_compiler);
-    let _ = fs::remove_file(&compiler);
-    copy(&rustc, &compiler);
-
-    // See if rustdoc exists to link it into place
-    let rustdoc = exe("rustdoc", host);
-    let rustdoc_src = out_dir.join(&rustdoc);
-    let rustdoc_dst = bindir.join(&rustdoc);
-    if fs::metadata(&rustdoc_src).is_ok() {
-        let _ = fs::remove_file(&rustdoc_dst);
-        copy(&rustdoc_src, &rustdoc_dst);
+        let out_dir = build.cargo_out(&build_compiler, Mode::Librustc, host);
+
+        // Link the compiler binary itself into place
+        let rustc = out_dir.join(exe("rustc", host));
+        let bindir = sysroot.join("bin");
+        t!(fs::create_dir_all(&bindir));
+        let compiler = build.compiler_path(&target_compiler);
+        let _ = fs::remove_file(&compiler);
+        copy(&rustc, &compiler);
+
+        // See if rustdoc exists to link it into place
+        let rustdoc = exe("rustdoc", host);
+        let rustdoc_src = out_dir.join(&rustdoc);
+        let rustdoc_dst = bindir.join(&rustdoc);
+        if fs::metadata(&rustdoc_src).is_ok() {
+            let _ = fs::remove_file(&rustdoc_dst);
+            copy(&rustdoc_src, &rustdoc_dst);
+        }
     }
 }
 
@@ -574,160 +870,6 @@ fn add_to_sysroot(sysroot_dst: &Path, stamp: &Path) {
     }
 }
 
-//// ========================================================================
-//// Build tools
-////
-//// Tools used during the build system but not shipped
-//// "pseudo rule" which represents completely cleaning out the tools dir in
-//// one stage. This needs to happen whenever a dependency changes (e.g.
-//// libstd, libtest, librustc) and all of the tool compilations above will
-//// be sequenced after this rule.
-//rules.build("maybe-clean-tools", "path/to/nowhere")
-//     .after("librustc-tool")
-//     .after("libtest-tool")
-//     .after("libstd-tool");
-//
-//rules.build("librustc-tool", "path/to/nowhere")
-//     .dep(|s| s.name("librustc"))
-//     .run(move |s| compile::maybe_clean_tools(build, s.stage, s.target, Mode::Librustc));
-//rules.build("libtest-tool", "path/to/nowhere")
-//     .dep(|s| s.name("libtest"))
-//     .run(move |s| compile::maybe_clean_tools(build, s.stage, s.target, Mode::Libtest));
-//rules.build("libstd-tool", "path/to/nowhere")
-//     .dep(|s| s.name("libstd"))
-//     .run(move |s| compile::maybe_clean_tools(build, s.stage, s.target, Mode::Libstd));
-//
-/// Build a tool in `src/tools`
-///
-/// This will build the specified tool with the specified `host` compiler in
-/// `stage` into the normal cargo output directory.
-pub fn maybe_clean_tools(build: &Build, stage: u32, target: &str, mode: Mode) {
-    let compiler = Compiler::new(stage, &build.build);
-
-    let stamp = match mode {
-        Mode::Libstd => libstd_stamp(build, &compiler, target),
-        Mode::Libtest => libtest_stamp(build, &compiler, target),
-        Mode::Librustc => librustc_stamp(build, &compiler, target),
-        _ => panic!(),
-    };
-    let out_dir = build.cargo_out(&compiler, Mode::Tool, target);
-    build.clear_if_dirty(&out_dir, &stamp);
-}
-
-
-// rules.build("tool-rustbook", "src/tools/rustbook")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("librustc-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "rustbook"));
-// rules.build("tool-error-index", "src/tools/error_index_generator")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("librustc-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "error_index_generator"));
-// rules.build("tool-unstable-book-gen", "src/tools/unstable-book-gen")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "unstable-book-gen"));
-// rules.build("tool-tidy", "src/tools/tidy")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "tidy"));
-// rules.build("tool-linkchecker", "src/tools/linkchecker")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "linkchecker"));
-// rules.build("tool-cargotest", "src/tools/cargotest")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "cargotest"));
-// rules.build("tool-compiletest", "src/tools/compiletest")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libtest-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "compiletest"));
-// rules.build("tool-build-manifest", "src/tools/build-manifest")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "build-manifest"));
-// rules.build("tool-remote-test-server", "src/tools/remote-test-server")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "remote-test-server"));
-// rules.build("tool-remote-test-client", "src/tools/remote-test-client")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "remote-test-client"));
-// rules.build("tool-rust-installer", "src/tools/rust-installer")
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .run(move |s| compile::tool(build, s.stage, s.target, "rust-installer"));
-// rules.build("tool-cargo", "src/tools/cargo")
-//      .host(true)
-//      .default(build.config.extended)
-//      .dep(|s| s.name("maybe-clean-tools"))
-//      .dep(|s| s.name("libstd-tool"))
-//      .dep(|s| s.stage(0).host(s.target).name("openssl"))
-//      .dep(move |s| {
-//          // Cargo depends on procedural macros, which requires a full host
-//          // compiler to be available, so we need to depend on that.
-//          s.name("librustc-link")
-//           .target(&build.build)
-//           .host(&build.build)
-//      })
-//      .run(move |s| compile::tool(build, s.stage, s.target, "cargo"));
-// rules.build("tool-rls", "src/tools/rls")
-//      .host(true)
-//      .default(build.config.extended)
-//      .dep(|s| s.name("librustc-tool"))
-//      .dep(|s| s.stage(0).host(s.target).name("openssl"))
-//      .dep(move |s| {
-//          // rls, like cargo, uses procedural macros
-//          s.name("librustc-link")
-//           .target(&build.build)
-//           .host(&build.build)
-//      })
-//      .run(move |s| compile::tool(build, s.stage, s.target, "rls"));
-//
-
-/// Build a tool in `src/tools`
-///
-/// This will build the specified tool with the specified `host` compiler in
-/// `stage` into the normal cargo output directory.
-pub fn tool(build: &Build, stage: u32, target: &str, tool: &str) {
-    let _folder = build.fold_output(|| format!("stage{}-{}", stage, tool));
-    println!("Building stage{} tool {} ({})", stage, tool, target);
-
-    let compiler = Compiler::new(stage, &build.build);
-
-    let mut cargo = build.cargo(&compiler, Mode::Tool, target, "build");
-    let dir = build.src.join("src/tools").join(tool);
-    cargo.arg("--manifest-path").arg(dir.join("Cargo.toml"));
-
-    // We don't want to build tools dynamically as they'll be running across
-    // stages and such and it's just easier if they're not dynamically linked.
-    cargo.env("RUSTC_NO_PREFER_DYNAMIC", "1");
-
-    if let Some(dir) = build.openssl_install_dir(target) {
-        cargo.env("OPENSSL_STATIC", "1");
-        cargo.env("OPENSSL_DIR", dir);
-        cargo.env("LIBZ_SYS_STATIC", "1");
-    }
-
-    cargo.env("CFG_RELEASE_CHANNEL", &build.config.channel);
-
-    let info = GitInfo::new(&dir);
-    if let Some(sha) = info.sha() {
-        cargo.env("CFG_COMMIT_HASH", sha);
-    }
-    if let Some(sha_short) = info.sha_short() {
-        cargo.env("CFG_SHORT_COMMIT_HASH", sha_short);
-    }
-    if let Some(date) = info.commit_date() {
-        cargo.env("CFG_COMMIT_DATE", date);
-    }
-
-    build.run(&mut cargo);
-}
-
-
 // Avoiding a dependency on winapi to keep compile times down
 #[cfg(unix)]
 fn stderr_isatty() -> bool {